What is the Harvest Moon and Why Is It Late This Year?

On Thursday, October 5, the moon will once again be full, with all of its sunlit side visible from Earth. Usually, October's full moon is known as the Hunter's Moon, in honor of deer-hunting season. But this year, thanks to a quirk in celestial schedules, October's full moon is instead the Harvest Moon.

That's because in the northern hemisphere, "Harvest Moon" is the term reserved for whatever full moon occurs closes to the autumn equinox. Most years, that moon falls in September, since the equinox is on or around September 22. But this year, September's early full moon, visible on September 6, was dubbed the Corn Moon instead, and the moniker "Harvest Moon" was held for October. A Harvest Moon last fell in October eight years ago, although it will happen again just three years from now.

Last October's full moon was known as the Hunter's Moon, but when this year's rises on Thursday it will be the Harvest Moon. Frazer Harrison/Getty Images

The late Harvest Moon was caused by the moon being new, with its sunlit side turned fully away from Earth, just two days before the equinox, on September 20. That was the first new moon after the one that caused August's total solar eclipse. September's new moon was also the beginning of a new year in the Jewish and Islamic traditions, which both run on lunar cycles.

North America's moon nicknames are adapted from American Indian terms for the moons. Tribes tied each lunar cycle to what was happening in the natural world. Most of the names now in popular use were coined by members of the Algonquin tribes, who once lived throughout New England and the Great Lakes area.

Other tribes across the continent have other nicknames for the moons—October names include "moon when the wind shakes off the leaves" among the Lakota people and "the moon the birds fly south" among the Cree people, according to Phil Konstantin, a retired NASA scientist with Cherokee roots who collects American Indian celestial terms.

In the southern hemisphere, full moons are also often given nicknames tied to seasonal events in the natural world, but because September's equinox there marks the beginning of the spring, October names instead include "Egg Moon," "Seed Moon," and "Pink Moon."

This year's Harvest Moon is the 10th of 12 full moons this calendar year. The next full moon, due on November 4, will be called the Beaver Moon. Later in November, the night sky will stay interesting thanks to Venus and Jupiter being visible unusually close together.